This invention relates to water treatment and, more particularly, to a method and apparatus for controlling the dosage rate of coagulant added to water to maintain a desired turbidity.
The coagulation process is used in water treatment to remove nonsettleable solids, particles otherwise too small to be easily removed by sedimentation or filtration. In coagulation, these nonsettleable solids are converted into larger and heavier settleable solids by adding and mixing chemical coagulants such as alum into the raw water. The settleable solids can then be removed by conventional flocculation, clarification, and filtration processes. Typically, the coagulant is added to the raw water or influent before it enters the main portion of a water treatment plant that includes the above processes.
Prior methods for controlling the coagulant dosage rate have varied. A simple method employed by many water treatment plants is to measure the turbidity of the treated water or effluent and manually adjust in response to the measurement the dosage rate of a coagulant pump dispensing coagulant into the water before it is filtered. More sophisticated methods employ an automatically controlled pump that adjusts the dosage rate based on electrical signals from a turbidity meter. Examples of such prior methods are disclosed in U.S. Pat. Nos. 3,605,775; 3,618,766; 3,693,797; and 3,731,807. U.S. Pat. No. 3,605,775, for example, discloses a method for controlling dosage that uses an influent turbidity meter and an effluent turbidity meter. The influent turbidity meter provides immediate control of the dosage rate. Its measurement signal is continuously compared to the output signal of the effluent turbidity meter for error correction. U.S. Pat. No. 3,693,797 also measures the turbidity of the water before and after material has been added for filtering. The relative values of the measured turbidities are use to control the addition of the material. Similarly, U.S. Pat. No. 3,618,766 discloses a method for controlling the addition of filter aid chemicals to water which relies on simultaneous measurements of turbidity at an intermediate point in the filter and at the final filter effluent. U.S. Pat. No. 3,731,807 discloses the use of multiple vessels each with a predetermined quantity of liquid and reagent and a turbidity meter for determining the optimum proportion of reagent to be added to the liquid. In all of these methods the level of effluent turbidity must be considered because regulatory agencies rely on effluent turbidity as a measure of water quality.
A problem with measuring effluent turbidity to set the coagulant dosage rate is the lag time between the addition of the coagulant dosage to the raw water and the measurement of the resultant turbidity. The relatively long detention time of the water in passing through the treatment plant makes accurate tracking of changes in raw water turbidity difficult. A sudden change in the turbidity of the influent, for example, will not be sensed by the turbidity meter at the output of the water treatment plant until the water has actually undergone treatment, too late to adjust the dosage rate. If the change is brief, then the dosage rate will overcompensate or undercompensate for an additional time. Several of the methods disclosed in the above-mentioned patents attempt to overcome this problem by measuring the turbidity at other points in the water treatment. However, measuring turbidity after the coagulant dosage is added and mixed but before water treatment is not an accurate measure of the required dosage for a desired effluent turbidity. Turbidity generally does not correlate to the coagulant needed for effective filtration because of differences in particle size.
Because of the lag time between dosage and measurement of its effect on effluent turbidity, plant operators typically overdose the raw water with coagulant as a safety precaution. Not only is this uneconomical, but the overdose can adversely affect the quality of water leaving the water treatment plant. For example, tap water treated with excessive alum contains soluble aluminum which can contribute to serious health problems. Excessive alum can also affect plant operation by reducing filter run times and raising backwash and pumping costs.
Recently devices have been introduced that measure the net ionic and colloidal surface charge of the suspended particles rather than turbidity. This net charge is a function of the balance of free positive and negative charges after coagulation and therefore varies proportionally with the concentration of free particles in a test cell. By measuring the net electrical charge, such devices can accurately detect changes in the concentration of the nonsettleable particles which are affected by the coagulant. And by relying on net electrical charge present after coagulation rather than effluent turbidity after water treatment to control the coagulant dosage, the devices can be located adjacent to the coagulant pump to detect and correct for changes in the influent turbidity immediately. An example of such a device is a streaming current detector available from Milton Roy Company of Ivyland, Pa. In common with many of such charge sensing or measuring devices, the detector has an adjustable gain and zero offset that must be calibrated to the desired level of effluent turbidity with a turbidity meter or other turbidity measuring means. The detector may be combined with a conventional controller to automatically control the dosage rate of a coagulant pump. When used with a controller, the streaming current detector is initialized to a desired net charge value (such as a neutral charge or 0 reading) and the controller is then programmed to maintain that charge value as a set point by adjusting the dosage rate of the coagulant pump. A detected increase in particle concentration produces a positive reading while a decrease in particle concentration prompts a negative reading. The controller responds to the reading to change the dosage rate of the coagulant pump accordingly.
Although these charge measuring devices are a great improvement over earlier methods of dosage control, they have their own drawbacks. For example, they are subject to measurement variation because of fouling of electrodes, amplifier drift, variations in the sample pH, and variations in the sample temperature. Over time, these variations can erroneously change the net charge value measured by the detector and thus cause the controller to adjust the coagulant dosage rate to maintain the set point. The conventional manner of correcting the device is to periodically clean or replace it. Such procedures, however, are costly and require suspension of plant operation.